The geology of California is highly complex, with numerous mountain ranges, substantial faulting and tectonic activity, rich natural resources and a history of both ancient and comparatively recent intense geological activity. The area formed as a series of small island arcs, deep-ocean sediments and mafic oceanic crust accreted to the western edge of North America, producing a series of deep basins and high mountain ranges.
One of the most important events was the advent of the San Andreas Fault around 29 million years ago in the Oligocene, when the region subducted a spreading center in the East Pacific Rise. This produced major crustal stretching, volcanism and displacement of up to .
Geological history The oldest rocks in California date back 1.8 billion years to the Proterozoic and are found in the San Gabriel Mountains, San Bernardino Mountains, and Mojave Desert. The rocks of eastern California formed a shallow continental shelf, with massive deposition of limestone during the Paleozoic, and sediments from this time are common in the Sierra Nevada, Klamath Mountains and eastern Transverse Range.
Active subduction began in the Triassic during the Mesozoic, producing large granite intrusions and the beginning of the Nevadan Orogeny as well as more dryland conditions and the retreat of the ocean to the west. Throughout the Jurassic the Nevadan Orogeny accelerated with large-scale granitic intrusions and erosion into deep marine basins. These basins steadily filled with sediment, with one famous example preserved as the Great Valley beds in the Coast Ranges. Simultaneously island arcs and small sections of continental crust rafted onto the edge of North America, building out the continent.
During the Cenozoic, the volcanic and deep water sedimentary Franciscan rocks were accreted to the edge of California and vast areas of marine sedimentary rocks deposited in the Central Valley and what would become the Transverse and Coast Ranges. Examples of filled basins included the Los Angeles Basin, the Eel River Basin around Eureka or the thick sedimentary sequences of the Ventura Basin. The San Andreas Fault became perhaps most active after the Miocene, potentially resulting in up to of offset in some locations.
Regional geology California Coast Ranges
The California Coast Ranges extend from the Oregon line southward to the Santa Ynez River. The mountains are drained by the Russian, Eel, Mad and Klamath rivers which tend to follow faults and folds. Per square mile of drainage, the Eel River has the highest suspended sediment load of any river in the US, exceeding the Colorado River and Mississippi River.
The Franciscan subduction complex makes up the basement rock for part of the range, with a mélange dominated by graywacke first deposited in offshore deep ocean basins. These rocks are rich in plagioclase, quartz and chlorite mica (which gives them a greenish color) and range up to thick. In some places, they are interbedded with shale, limestone and radiolarian chert. Examples of rock formations in the Franciscan subduction complex include the Calera limestone east of the San Andreas fault or the Laytonville limestone which extends north of San Francisco to Eureka.
Franciscan rocks have a complex geological structure. In some places, ophiolite sequences preserve remnant oceanic crust rock on land, while other parts of the range are intruded with ultrabasic igneous rocks that have serpentinized to peridotite.
In the southern part of the range, between the Nacimiento and San Andreas fault zones, the metamorphic rocks and granite plutons of the Salinian block make up the basement rock. These rocks were likely moved to their current location by the two major faults. Some Great Valley rocks are exposed in the Coastal Ranges as well, like the Jurassic Knoxville Formation shale.
Sur Series schist, quartzite, marble, gneiss and granulite in the Salinian block were likely deposited in a marine shelf late in the Paleozoic. Shelf and slope deposits would ultimately become Great Valley rocks, while the trench deposits became the Franciscan subduction complex.
Major geologic changes began in the Cenozoic with continued continental shelf deposition of shale, sandstone and clay as well as near-shore tropical coal deposits. During the Miocene, the Coast Ranges were flooded again, with underwater volcanic eruptions in the southern part of the range and deposition of fossil-rich shales like the Monterey formation. Uplift took place into the Pliocene and Pleistocene, slowly reducing sea levels inland in the Central Valley.
The Coast Range is at even higher risk for damaging landslides than other parts of coastal California due to sheared serpentinite in Franciscan basement rocks.
Central Valley
The Central Valley includes the southern San Joaquin Valley and the northern Sacramento Valley. The Great Valley Sequence is a thick formation at the western edge of the Central Valley that formed between the Jurassic and Cretaceous, overlying the Franciscan Assemblage and granite rocks associated with the Sierra Nevada to the east. It represents an ancient forearc basin that took shape as oceanic crust subducted under the west edge of the continent.
During the Jurassic, many sediments shed into the region from the rising proto-Sierra Nevada. For much of the Cenozoic the region was filled with lakes and brackish swamps. Thick Miocene sediments formed in narrow seaways from the Pacific. The Stockton fault and White Wolf fault by Bakersfield are both major tectonic features. The 7.6 magnitude 1952 Kern County earthquake was one of the most powerful in California in the 20th century. The subsurface is well known from oil wells and oil fields are bounded in the east by the Kern Front fault. A small andesitic dome near Marysville is the only example of volcanic rocks in the valley exposed near the surface.
Coronado Island
The distinctive Coronado Island in San Diego County helps to create the large sheltered harbor of San Diego. It formed as a sandspit filled in to the north from sediment deposited by the Tijuana River and sheltered from wind and waves by the Cretaceous uplands of Point Loma. Two small islands to the north: North Island and South Island have been connected with fill to create the Naval Air Station. Without continuous dredging, San Diego Bay would fill with sediment and become dry land with deposition from the San Diego, Sweetwater and Otay rivers.
Klamath Mountains
The Klamath Mountains formed as small island arc terranes accreted against the coast of North America before and during the Devonian and overlain by newer rocks, deposited from the Cretaceous to modern times. Geologists have subdivided the four terranes in the mountain range, referring to them as "plates."
The Klamath Mountains have several areas of preserved oceanic crust. The Josephine ophiolite forms the basement of the western Klamath Mountains, with extensive peridotite from the boundary between the oceanic crust and the mantle, as well as chromite and nickel deposits. It is exposed in the Smith River drainage to the north of US Highway 199 in Del Norte County. By contrast, the Trinity ophiolite is found in the east, known for its numerous ultramafic rocks and being one of the most extensive ophiolites in the US. The Western Jurassic Plate stretches for on the western edge of the mountains. The thick Galice formation slate and metamorphosed greywacke overlie the Josephine ophiolite and likely deposited in a deep ocean environment offshore of the island arc. It also interfingers with up to of volcanic rocks in the Rogue Formation. Some rocks in the Western Jurassic Plate seem to have been subducted, reaching blueschist grade metamorphism in the sequence of metamorphic facies in the South Fork Mountain schist.
By contrast, the Eastern Klamath Plate is made up of limestone, chert and pyroclastic rocks, divided in half by the Trinity ophiolite. A thrust fault, known as the Trinity or Bully Choop thrust, separates the Eastern Klamath Plate from the Central Metamorphic Plate. This region of the mountains was likely underthrusted by the adjoining plates, producing multiple schists around 380 to 400 million years ago in the Ordovician. Many of these schists, like the Condrey Mountain schist are more resistant to erosion than other rocks close by. The Western Paleozoic and Triassic Plate is the most common unit of the Klamath Mountains and is up to wide at the Oregon state line. Geologists have struggled to define its structural geology with complex sequences of deep ocean crust, upper mantle rock and tectonic melanges.
Granite plutons emplaced during the Jurassic and Cretaceous, forming rock units like the Ironside Mountain diorite, which outcrops for from the Orleans Mountain lookout tower to the line between Humboldt County and Siskiyou County or the Shasta Bally batholith at Buckhorn Summit west of Redding. A few may date earlier, like the 400 million year old Mule Mountain. During the Pleistocene, the mountains had a large number of glaciers and cirques and glacially-carved U-shaped valleys are remaining features from that time period. Large boulders have often eroded out of moraine deposits, coming to rest a few miles away. Fast flowing rivers meant that the region has accumulated very little alluvium, except for a rare thick deposit in Scott Valley, southwest of Yreka, California.
Modoc Plateau
The Modoc Plateau is an undulating expanse of Miocene to Holocene basalt law flows at the southwest edge of the Columbia Plateau, covering . It is drained by the Pit River, which ultimately reaches Shasta Lake and the Sacramento River. The Warner Basalt is the most common rock in the plateau, bordered by the Surprise Valley fault zone that first became active in the Miocene 15 million years ago. Based on faulting in recent alluvial material, the Surprise Valley fault zone is still active.
In spite of low rainfall, the numerous lava tubes and volcanic fractures in the plateau produce the Fall River Springs, one of the largest springs in the US. At least 300 lava tubes are known in Lava Beds National Monument, some of which preserve ice year round.
The rugged terrain of the Modoc Plateau played an important role in the Modoc War. In 1873, 53 Modoc men held off 650 US troops, killing 70 of them in heavy weather in the lava plateau to the south of Tule Lake. The Modoc exploited collapse pits and lava tunnels, turning to Tule Lake for food (subsequent drainage has reduced the lake shoreline).
Mojave Desert
The Mojave Desert extends into areas of Basin and Range terrain and includes some of the oldest rocks exposed at the surface in California. The movement of the San Andreas Fault and Garlock Fault helped to induce arid conditions during the Cenozoic. Unlike most of California, the Mojave region has numerous Precambrian Proterozoic rocks, such as granitic gneiss or marble intruded with porphyry in the Ord Mountains or schist in the Old Woman Mountains.
The Marble Mountain Cambrian quartzite lies unconformably atop Proterozoic granites. During the Jurassic high sea levels retreated, switching to the deposition of the terrestrial Aztec sandstone. Volcanic rocks erupted around Barstow in the Triassic. Lake beds interbedded with volcanic ash are typical of the Cenozoic. Volcanism continued through the Holocene, forming the 300 foot thick basalts of the Cima Volcanic Field or the Barstow-Amboy axis of volcanic craters, which have protected underlying granite from erosion. Unique geology in the region formed bastnaesite mined at the Mountain Pass rare earth mine. Additionally, borate deposited during arid conditions, with mining after 1926.
Newport Bay